Several publications and patent documents are cited throughout the specification in order to describe the state of the art to which this invention pertains. Each of these citations is incorporated herein by reference as though set forth in full.
The agricultural industry is responsible for providing food for an ever growing global population. Currently, population growth is on track to outpace agricultural growth by the year 2050 (OECD and FAO, 2012; Ray et al., 2013; Tilman et al., 2011), necessitating the development of new technologies to increase agricultural production. This challenge is compounded by climate change, which is reducing arable lands that can be used for crop production (Olesen and Bindi, 2002; Rosenzweig and Parry, 1994). Clearly, there is a need to develop plants that can better withstand drought conditions and nutrient-poor soils without compromising vegetative, fruit, or seed production. One method to achieve this is through the study of plant root development, as roots function in the uptake of both water and nutrients from the environment (Grierson et al., 2014; Hofer, 1991). Thus, these studies can result in the engineering of plants that can better tolerate and respond to these environmental stresses, without affecting the development of the agriculturally important aerial tissues.
The plant root epidermis is responsible for absorbing both water and nutrients from the environment (Grierson et al., 2014; Hofer, 1991). During root growth, epidermal precursor cells differentiate (Cormack, 1935, 1949; Dolan et al., 1993) into either root hair or non-hair cells. The long hair-like projections of hair cells dramatically increase surface area, allowing uptake of more nutrients from the surrounding soil. Therefore, plants regulate the ratio of root hair to non-hair cells in a manner that is partially dependent on environmental signals (Bates and Lynch, 1996; Ma et al., 2001; Meisner and Karnok, 1991). More specifically, plants grown under nutrient or water poor conditions develop more hair cells with longer hairs (Bates and Lynch, 1996), thereby greatly increasing the surface area of the root to promote increased absorption.
Phosphate limitation is one of the most common nutrient stresses that plants face when growing in fields for agriculture production. This is because roots can only absorb inorganic phosphates, which are naturally present at very low concentrations in soil (Heckrath et al., 1995; Patrick and Khalid, 1974).
Therefore, plants have developed numerous mechanisms by which to maximize the uptake of this nutrient in phosphate poor soil (Gahoonia et al.; Lynch and Brown; Niu et al., 2013; Williamson et al., 2001). In fact, researchers have described three major changes in Arabidopsis thaliana (hereafter Arabidopsis) root development during phosphate starvation. First, the primary root ceases downward growth, with a subsequent increase in lateral roots branching away from primary roots (Linkohr et al., 2002; Reymond et al., 2006; Williamson et al., 2001). Additionally, the root epidermis dramatically increases the number of root hair cells, while also increasing their length (Bates and Lynch, 1996). Finally, root epidermal cells secrete acid phosphatases, enzymes that catalyze organic into inorganic phosphates, which can be subsequently absorbed (Gilbert et al., 1999; Tadano et al.). Thus, there is a clear link between response to phosphate starvation and root hair cell fate. However, the molecular mechanisms by which exogenous phosphate levels regulate this cell fate decision are not fully understood.